The development of satellite tracking technology in the 1950's and atomic clock technology in the 1960's and 1970's led to the development of navigation (and global positioning) systems using satellite and clock signals. A generalized method of satellite navigation and global positioning is shown with reference to FIG. 1, wherein a satellite 10 is shown at a known distance R.sub.S from the center C of the Earth 12. FIG. 1 also shows a global positioning system (GPS) receiver location 14 on the Earth's surface, at a distance R.sub.E from the center C.
The distances R.sub.S and R.sub.E are known or can be determined at least within a few meters. The range X is the distance from the receiver location 14 and the satellite 10, which can be measured and calculated electronically from satellite transmitted signals. This measuring and calculating process involves the use of synchronized clocks, one at the satellite 10 and one at the receiver location 14 and the transmission of a clock-based signal from the satellite for comparison with a replicate clock-based signal from the synchronized clock at the receiver location 14. An estimate of the range X can be determined from the phase difference between the signal received from the satellite and the signal at the receiver location.
Because the R.sub.S and R.sub.E sides of the triangle shown in FIG. 1 are known, once the range X is calculated, the solved range defines a range line (or line-of-position) on the surface of the Earth upon which the receiver location 14 must be positioned. At least two of such lines are needed to determine a two-dimensional identification (or fix) of the receiver location 14 on the Earth's surface, using the range line approach. At least three of such lines are needed to determine a third dimension, such as altitude or depth. However, this generalized example assumes that the range X is calculated with perfect accuracy. In real-world applications, on the other hand, range calculations require a substantial amount of calculations to account for a number of variables, including Doppler effects, receiver clock imprecisions, and atmospheric delays.
Clock errors may be estimated by forming a third range line for a two-dimensional fix or a fourth range line for a three-dimensional fix. This can be demonstrated, for example, with reference to FIG. 2, wherein three satellites are used to provide an estimated two-dimensional fix and an estimated clock error.
In FIG. 2, the pseudo-range X of three satellite positions is illustrated as pseudo-range radii R.sub.X1, R.sub.X2 and R.sub.X3. The pseudo-range radii R.sub.X1, R.sub.X2 and R.sub.X3 do not meet in a single point, but instead enclose the shaded region shown in FIG. 2. However, a value .DELTA.r can be determined that, when removed from (or added to) the pseudo-ranges, will cause the radii to meet at a point 16. If the error between the user's clock and the space vehicle clock is the same for each satellite and if other effects on range determinations, such as Doppler effects, are the same for each satellite, then the point 16 is the user's position. However, in practice, the clock errors and Doppler effects differ for different satellites and satellite positions such that the value .DELTA.r is actually an estimate of the range error caused by clock error and the position 16 is actually an estimate of the user's position, based on these estimated error values.
In general, the accuracy of the position determination calculations tend to increase as more range lines are formed. In a single satellite arrangement, multiple range lines are formed over a period of time, as the range X changes with relative motion between the satellite and the location 14. Multiple satellites (for example, the NAVSTAR system) can increase the processing speed by providing the ability to obtain multiple range lines at any one time.
For example, NAVSTAR includes multiple satellites positioned in respective known orbits, such that from 6 to 11 satellites are visible at 5 degrees or more above the horizon to a user located anywhere in the world. Atomic clock technology employed in the satellites provides precision timing for satellite transmitted signals. Each satellite transmits a signal modulated with a unique spread spectrum code for timing, identification, and other data such as status data and parameters for clock correction.
GPS receivers are typically designed to search for and identify all NAVSTAR satellites within range for making navigational calculations. At least four are generally required for three-dimensional tracking, as discussed above. However, with multiple satellite signals, the receiver complexity (and time) required to discriminate and identify each satellite's signal, and account for Doppler effects, clock error and other error factors increases significantly. Acquisition of the requisite number of satellites can involve relatively complex calculation processes dependent upon navigation calculations and satellite signal measurements. In typical conventional receivers, navigation and measurement functional parts are intertwined, in that operation of the receiver requires that the measurement engine be provided data from the navigation engine for continued satellite acquisitions.
One example of a global positioning method and apparatus for a GPS receiver system is described in co-assigned U.S. Pat. No. 5,552,794 to Colley et al., issued Sep. 3, 1996 (incorporated herein by reference). Systems such as described in the Colley et al. patent require both navigation and measurement means (engines), which are operationally dependent upon each other, to acquire the requisite satellite signals to determine (or fix) a user's position. In particular, as each satellite signal is acquired, Doppler measurements are made to determine the rate of change of the range. Navigation calculations are carried out to determine navigational coordinates representing a corresponding "isodop line" over the globe (a line of positions at which the range rate of change, or Doppler measurements, are constant), defining hypothetical receiver positions along the line. By placing the receiver's hypothetical position on various points along the isodop, other satellites that may be visible from the isodop points may be acquired to determine further isodop lines.
This interdependent operation of navigation calculations and measurement functions can provide relatively accurate position calculations, however, at the cost of significant computational complexity and time. In many potential satellite positioning system applications, it would be desirable to minimize complexity, for purposes such as minimizing cost, power consumption, or size. Minimized computational complexity and time would benefit a number of potential fields of applications for satellite positioning systems, including, but not limited to, animal or personnel locating and tracking systems, water current and tide tracking systems, vehicle locating and tracking system, inventory or package locating and tracking systems or the like.